It is well known that for building tunnels, particularly when the excavation of the tunnel is to be effected in soft soils which walls are not self supporting and must be provided with a casing during excavation, and still more particular when excavating through non cohesive soils or sands both above or below subsurface water and through muddy soils or soft clays of any type, as well as soils formed by gravel or boulders, excavation methods are used in which a casing or supporting tube is arranged within the excavation in order to prevent collapsing of the walls of the tunnel, inasmuch as the latter are not self supporting.
A vast plurality of methods to accomplish the above have been deviced and are very well known in the art. Some typical processes comprise, inter alia, the provision of pneumatic chambers and the use of compressed air to fill the excavation and avoid collapsing thereof, but the above so called pressure systems have not gained popularity because they are unsafe, costly and require the use of highly sophisticated equipment. Other typical and more widely used processes are those using the so called "push-casing" systems, wherein a casing is arranged to be pushed by means of jacks or the like, with its outer surface in direct contact with the excavation and with the application of a suitable lubricating agent to facilitate displacement of the casing by reducing the friction between the outer surface of the casing and the surface of the excavation.
As lubricating agents for the above purpose, bentonite paste type lubricants are generally used, but these lubricants are simply applied in very thin layers between the sliding parts, which merely serve to avoid as much as possible the friction stresses, but without achieving the goal of reducing friction forces to an extent suitable to enable pushing inwardly of the excavation large lengths of casing or large diameter casings.
Therefore, prior art processes, particularly those which are effected by pushing a casing inwardly of the excavation, have not been adequate to permit insertion of a casing having a length or a diameter sufficiently large and, therefore, such excavation methods are restricted to relatively small tunnels, inasmuch as the pushing forces become excessively large when the length or the diameter of the excavation are large.
In view of the above, for excavating tunnels of considerable length or diameter, the pushing method has proven to be quite impractical, whereby other processes for supporting the walls of the excavation must be used, with the consequent increase in the costs of operation. The fact that the most economical method presently known for excavating tunnels, that is, the push-casing method, is impractical for excavating large tunnels, represents a serious drawback in the tunnel excavating art, but the fact is that said push-casing methods have not been sufficiently developed to reduce the friction forces between the casing and the walls of the excavation to an extent sufficient to permit pushing a large casing inwardly of said excavation.
Therefore, for long a process has been sought that, having the economical characteristics and advantages of the push-casing method, may also provide for the possibility of easily pushing large casings inwardly of straight or curved excavations, without the need of having resort to other costly methods for supporting the walls of the excavation.